In wireless communication, if multiple antennas are used by both a transmitter and a receiver to transmit and receive signals, then a higher data rate may be obtained by spatial multiplexing, that is, multiple data streams may be sent at the transmitter using the same time-frequency resource, a channel coefficient matrix may be obtained at the receiver through channel estimation, and then data in each data stream may be demodulated.
Spatial multiplexing includes open-loop spatial multiplexing and close-loop spatial multiplexing. FIG. 1 is a schematic diagram of a flow of MIMO signal processing by close-loop spatial multiplexing in related art. As shown in FIG. 1, with close-loop spatial multiplexing, a transmitter performs pre-coding on a signal according to Channel State Information (CSI); here, one way for the transmitter to acquire CSI is to acquire CSI through feedback by the receiver. Generally, in order to reduce the overhead of the feedback, the receiver and the transmitter save the same codebook, that is, a pre-coding matrix set; the receiver selects a proper pre-coding matrix from the codebook according to current channel condition, and feeds back, to the transmitter, a Pre-Coding Matrix Indicator (PMI) of the selected pre-coding matrix in the pre-coding matrix set; then, the transmitter finds the pre-coding matrix according to the PMI feed back, and performs pre-coding on the signal to be sent. With open-loop spatial multiplexing, the transmitter does not perform pre-coding on the signal according to the CSI sent by the receiver, but according to a predetermined fixed codebook combination.
In the next-generation evolution of a Long Term Evolution (LTE) system, that is, an LTE-Advanced (LTE-A) system, in order to obtain a higher data rate, Single User MIMO (SU-MIMO) techniques are adopted for an uplink of the LTE-A system, wherein a terminal serves as a transmitter and a base station serves as a receiver, and the direction from the terminal to the base station is an uplink direction. The SU-MIMO techniques belong to close-loop spatial multiplexing. FIG. 2 is a schematic diagram of signal processing at a transmitter adopting uplink SU-MIMO techniques; as shown in FIG. 2, coded bit sequences corresponding to codeword 0 and codeword 1 of an uplink signal of the terminal are scrambled and then modulated respectively to obtain complex symbols corresponding to the respective codewords; then, the codeword 0 and the codeword 1 forming the complex symbols are subjected to layer mapping to obtain data of layer 0 and of layer 1 respectively; next, the obtained data of the layers are subjected to transmission pre-coding so as to be converted from a time-domain signal into a frequency-domain signal; the signal is then subjected to pre-coding in the frequency domain, resource mapping, and then Single Carrier Frequency Division Multiple Access (SC-FDMA) symbol generation before finally being transmitted on the antennas. Here, the transmission pre-coding refers to Discrete Fourier Transform (DFT).
In the above method, a codeword-to-layer mapping module of the terminal implements the codeword-to-layer mapping by simple serial/parallel conversion. In the LTE-A system, a codeword-to-layer mapping mode as shown in the following table is adopted in the SU-MIMO.
Number ofNumber ofcodeword-to-layer Mappinglayerscodewordsi = 0, 1, . . . , Msymblayer − 111x(0)(i) = d(0)(i)Msymblayer = Msymb(0)21x(0)(i) = d(0)(2i)Msymblayer = Msymb(0)/2x(1)(i) = d(0)(2i + 1)22x(0)(i) = d(0)(i)Msymblayer = Msymb(0) =x(1)(i) = d(1)(i)Msymb(1)32x(0)(i) = d(0)(i)Msymblayer = Msymb(0) =x(1)(i) = d(1)(2i)Msymb(1)/2x(2)(i) = d(1)(2i + 1)42x(0)(i) = d(0)(2i)Msymblayer = Msymb(0)/2 =x(1)(i) = d(0)(2i + 1)Msymb(1)/2x(2)(i) = d(1)(2i)x(3)(i) = d(1)(2i + 1)
In the table, d(q)(0), d(q)(1), . . . , d(q)(Msymb(q)−1) each represents a modulated complex symbol corresponding to codeword q, qε{0,1}; Msymb(q) represents the number of modulated complex symbols corresponding to codeword q; x(υ)(0), x(υ)(1), . . . , x(υ)(Msymblayer−1) each represents a modulated complex symbol corresponding to layer υ, υε{0, 1, 2, 3}; Msymblayer presents the number of modulated complex symbols corresponding to one layer.
In order to obtain a higher transmission rate, the LTE-A system supports the configuration of four uplink sending antennas. Limited transmitting power of the terminal greatly affects uplink-transmission-technique selection. However, with multi-carrier techniques such as Orthogonal Frequency Division Multiplexing (OFDM), multiple independent sub-carriers are used together, and the signal to be sent has a very high Peak-to-Average Power Ratio (PAPR), which brings about many disadvantages, for example, increased complexity of analog-to-digital conversion and digital-to-analog conversion and reduced radio-power-amplifier efficiency, thereby increasing the cost and power consumption of a power amplifier of a transmitter. Due to limited cost and power consumption of a terminal, it is not advantageous to implement multi-carrier techniques on an uplink. Therefore, in order to ensure a low PAPR or Cubic Metric (CM) of the signal to be sent, in the LTE-A system, SC-FDMA is adopted in uplink transmission; specifically, Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM) is adopted. Here, the CM is an index for measuring nonlinear influence on the power amplifier, which index is more accurate than the PAPR.
As shown in FIG. 2, generally, in an uplink of the LTE-A system, data of a layer are subjected to transmission pre-coding before the pre-coding; in which case, in order to ensure that the signal to be sent has a low PAPR or CM, factors such as PARP or CM should be taken into account in designing the codebook of pre-coding matrices; therefore, for codebook design, there is one more constraint, for example, of adopting a CM Preserved (CMP) design. In a practical application, all pre-coding matrices ultimately adopted in LTE-A uplink close-loop spatial multiplexing ensure that the uplink signal to be sent has a lower PAPR or CM.
In the LTE-A system, since a typical scenario in application is of moderate/low mobility, multi-antenna techniques with open-loop spatial multiplexing are not supported. However, in a subsequent version of LTE-A, such as Release-11, a scenario of high mobility is reconsidered as a primary direction for optimization of the subsequent version. When a terminal moves with a high speed, for example, up to 350 km/h, due to a fast changing channel status, CSI-feedback-based close-loop spatial multiplexing would degrade system performance, in which case, it is more reasonable to adopt open-loop spatial multiplexing. In this case, if it is to adopt in uplink open-loop spatial multiplexing a sequence of signal processing similar to that in uplink close-loop spatial multiplexing, that is, pre-coding is conducted after transmission pre-coding, then in order to ensure that the signal to be sent has a low PAPR or CM, there also are constraints on the design and selection of the codebook of pre-coding matrices; therefore, there is a pressing need for improved open-loop multiplexing in a subsequent version of the LTE-A.